1. Field of the Invention
The present invention relates to a process for producing a photosensitive lithographic plate. In particular, the present invention relates to a process for producing a photosensitive lithographic plate wherein a negative working photosensitive composition is applied to a substrate by using a special solvent.
2. Description of the Prior Art
A process for producing a photosensitive lithographic plate by applying a photosensitive composition comprising a photosensitive diazonium salt and a linear organic polymeric compound to a substrate having a hydrophilic surface has been well known.
However, a photosensitive lithographic plate produced by applying a solution of the photosensitive composition to the substrate having the hydrophilic surface, particularly an aluminum plate, the surface of which is made hydrophilic by a treatment, has the defect that the developability thereof is reduced with time and a non-image area is stained in the printing step.
Various processes have been proposed for overcoming this defect. For example, Japanese Patent Unexamined Published Application (hereinafter referred to as "J. P. KOKAI") No. 62-38471, which corresponds to U.S. patent application, Ser. No. 06/896,025 filed on Aug. 13, 1986, discloses a photosensitive composition containing a photosensitive diazo resin and a polymeric compound wherein at least 10 wt.% of a solvent used is 1-methoxy-2-propanol so as to inhibit the reduction of developability with time.
In the production of the lithographic plates on an industrial scale, a photosensitive resin coating solution is continuously applied to a substrate conveyed in the form of a belt, then the coated substrate is dried to form a thin film having a uniform thickness. Therefore, optimum concentration and viscosity are required of the photosensitive resin coating solution. A coating solution containing a large amount of a solvent having a boiling point of as high as 100.degree. C. or higher is not preferred, since it requires a long, complicated drying device. A solvent having a boiling point of lower than 50.degree. C. is dangerous, since it easily ignites or explodes. Further, such a solvent is rapidly evaporated in the drying step, making the photosensitive layer uneven. It is preferred, therefore, to use a mixture of a low-boiling solvent having a boiling point of 50.degree. to 100.degree. C. and a high-boiling solvent having a boiling point of higher than 100.degree. C.
However, in a photosensitive composition in which the solvent comprises 1-methoxy-2-propanol containing at least 20 wt.% of such a low boiling solvent, the compatibility between the photosensitive diazo resin and the linear organic polymer is poor and sufficient printing durability cannot be obtained.